1. Field of the Invention
2. Description of the Related Art
The power generated by an internal combustion engine depends on the air mass and the fuel quantity which can be made available to the engine for combustion. In order to increase the power of an internal combustion engine, the quantity of combustion air and fuel supplied must be increased. In the case of a naturally aspirated engine, this power increase is achieved by an increase in swept volume or by increasing the engine speed. However, an increase in swept volume leads in principle to heavier engines of larger dimensions, which are therefore more expensive. The increase in engine speed entails considerable problems and disadvantages, especially in the case of relatively large internal combustion engines, and is limited for technical reasons.
A much-used technical solution for increasing the power of an internal combustion engine is boosting. This refers to precompression of the combustion air by an exhaust gas turbocharger or by means of a compressor mechanically driven by the engine. An exhaust gas turbocharger consists essentially of a flow compressor and a turbine which are connected by a common turboshaft and rotate at the same speed. The turbine converts the normally uselessly discharged energy of the exhaust gas into rotational energy and drives the compressor. The compressor aspirates fresh air and conveys the precompressed air to the individual cylinders of the engine. An increased quantity of fuel can be supplied to the larger air quantity in the cylinders, whereby the engine delivers more power. In addition, the combustion process is influenced favorably, so that the engine achieves better overall efficiency. Furthermore, the torque curve of an internal combustion engine boosted with a turbocharger can be configured extremely favorably. Existing naturally aspirated engines in series production by manufacturers can be significantly optimized by the use of an exhaust gas turbocharger without major interventions in engine design. Boosted internal combustion engines generally have a lower specific fuel consumption and lower pollutant emissions. Moreover, turbocharged engines are quieter than naturally aspirated engines of the same power because the exhaust gas turbocharger itself acts like an additional silencer.
For internal combustion engines with a wide operating speed range, for example, passenger car engines, a high boost pressure is required even at low engine speeds. For this purpose a boost pressure control valve, a so-called waste gate valve, is introduced with these turbochargers. Through the selection of a suitable turbine casing a high boost pressure is rapidly built up even at low engine speeds. The boost pressure control valve (waste gate valve) then limits the boost pressure to a constant value as engine speed rises. Alternatively, turbochargers with variable turbine geometry (VTG) are used. In these turbochargers the boost pressure is regulated by changing the turbine geometry.
With increased exhaust gas quantity, the maximum permissible speed of the combination of turbine wheel, compressor wheel and turboshaft, also referred to as the rotor of the turbocharger, may be exceeded. Impermissible exceeding of the speed of the rotor would destroy the latter, which is equivalent to total loss of the turbocharger. In particular modern, small turbochargers with significantly smaller turbine wheel and compressor wheel diameters, which have improved rotational acceleration behavior through a considerably lower mass moment of inertia, are affected by the problem of exceeding the permissible maximum rotational speed. Depending on the design of the turbocharger, exceeding of the rotational speed limit even by approximately 5% causes complete destruction of the turbocharger.
Boost pressure control valves which, according to the prior art, are activated by a signal resulting from the boost pressure generated, have proved effective for limiting rotational speed. If the boost pressure exceeds a predetermined threshold value, the boost pressure control valve opens and conducts a part of the exhaust gas mass flow past the turbine. Because of the reduced exhaust gas mass flow, the turbine absorbs less power and the compressor output is reduced proportionally. The boost pressure and the rotational speed of the turbine wheel and the compressor wheel are reduced. However, this regulation is relatively sluggish, because the pressure build-up in the event of the rotor exceeding a given speed occurs with a time offset. For this reason, regulation of turbocharger speed by monitoring boost pressure, especially in the high dynamic range (load change), must intervene by correspondingly early reduction of boost pressure, incurring a loss of efficiency.